1. Field of the Invention
The present invention generally relates to a transconductance amplifier, the transconductance of which can be changed when the polarity of a differential input voltage differs, and an automatic gain control device using it.
2. Description of the Related Art
The transconductance amplifier is such that different output currents can be obtained therefrom for a certain differential input voltage. FIG. 1 shows a circuit of such an amplifier (referred to as a `transconductance amplifier circuit`, hereinafter). The transconductance amplifier circuit will now be described briefly based on FIG. 1.
This transconductance amplifier circuit includes a plurality of p-channel MOS transistors Tr1, Tr2, Tr3, Tr7 and Tr8, and a plurality of n-channel MOS transistors Tr4, Tr5, Tr6 and Tr9, which are connected as shown in FIG. 1. Each pair of the transistors Tr4 and Tr6, transistors Tr7 and Tr8, and transistors Tr5 and Tr9 form a current-mirror circuit. The characteristics are the same between each pair of transistors which forms the current-mirror circuit.
In this transconductance amplifier circuit, a current I1 flows from a connection point 1 to a connection point 2, and, also, the same current I1 flows from the connection point 1 to a connection point 3. From an inverting input terminal `a` connected to the gate of the transistor Tr1 and a non-inverting input terminal `b` connected to the gate of the transistor Tr2, these transistors Tr1 and Tr2 constituting a differential stage, voltages are applied to the gates of these transistors Tr1 and Tr2, respectively. A current I2 flows from the connection point 2 to the source of the transistor Tr1, and, also, the same current I2 flows from the connection point 3 to the source of the transistor Tr2. Accordingly, a current (I1-I2) flows from the connection point 2 to a connection point 4, and, also, the same current (I1-I2) flows from the connection point 3 to a connection point 5. The voltage applied to the gate of the transistor Tr2 is higher than the voltage applied to the gate of the transistor Tr1 by Vin (referred to as a `differential input voltage Vin`, hereinafter). As a result, a current `i` depending on the voltage difference between the gate and source of the transistor Tr3 flows from the connection point 5 to the connection point 4. Accordingly, a current (I1-I2+i) flows from the connection point 4 to the drain of the transistor Tr4, and a current (I1-I2-i) flows from the connection point 5 to the drain of the transistor Tr5.
Because the transistors Tr4 and Tr6 form the current-mirror circuit as mentioned above, the same current (I1-I2+i) as the current flowing from the connection point 4 to the drain of the transistor Tr4 flows from the drain of the transistor Tr7 to the drain of the transistor Tr6. Further, because the transistors Tr7 and Tr8 form the current-mirror circuit as mentioned above, the same current (I1-I2+i) as the current flowing from the drain of the transistor Tr7 to the drain of the transistor Tr6 flows from the drain of the transistor Tr8 to the connection point 6. Furthermore, because the transistors Tr5 and Tr9 form the current-mirror circuit as mentioned above, the same current (I1-I2-i) as the current flowing from the connection point 5 to the drain of the transistor Tr5 flows from the connection point 6 to the drain of the transistor Tr9. Thereby, a current 2i which is the difference between the current (I1-I2+i) flowing from the drain of the transistor Tr8 to the connection point 6 and the current (I1-I2-i) flowing from the connection point 6 to the drain of the transistor Tr9 is output from the connection point 6 in the direction indicated by the arrow. Hereinafter, the direction of the output current Iout is referred to as being positive when the current flows in the direction indicated by the arrow, and the direction of the output current Iout is referred to as being negative when the current flows in the direction reverse to the direction indicated by the arrow.
In this transconductance amplifier circuit, it is possible to change the transconductance of this circuit by changing the voltage difference between the gate and source of the transistor Tr3. In other words, as a result of the voltage difference between the gate and source of the transistor Tr3 being changed, the current `i` flowing from the connection point 5 to the connection point 4 changes. Thus, it is possible to change the value of the current 2i output from the connection point 6, and, thus, to change the absolute value of the output current Iout, even when the value of the differential input voltage Vin does not change. Thus, it is possible to change the transconductance of this circuit.
In a transconductance amplifier circuit disclosed in International Patent Application, International Publication Number of which is WO 96/07927, a plurality of transistors are connected in parallel in an output stage, and, it is possible to change the absolute value of the output current and to change the transconductance of the circuit by selecting transistors which form current mirror circuits without changing the voltage difference between the gate and source of the transistor Tr3 shown in FIG. 1.
Such a transconductance amplifier circuit is used in such a form, as shown in FIG. 3, in which an integrator is added to the output of a transconductance amplifier (referred to as a `Gm-C integrator`, hereinafter), or is used in an automatic gain control device (a device provided with an automatic gain control circuit) which controls a gain of an apparatus using a Gm-C integrator. In such a use, there is a case where it is desired to change the absolute value of the output current when the polarity of the differential input voltage changes. This is because, in some apparatuses, the gain of which is controlled by an automatic gain control device, it is not always preferable to have a shorter setting time but it may be desired to change the setting time when the polarity of the differential input voltage changes, wherein the setting time originates in the value of the output current of the transconductance amplifier. In order to change the absolute value of the output current of the transconductance amplifier when the polarity of the differential input voltage changes, it is necessary to change the transconductance of the transconductance amplifier when the polarity of the differential input voltage changes.
However, in the above-described related art, although it is possible to change the transconductance, it is not possible to change the transconductance corresponding to the inclination of the line when the polarity of the differential input voltage changes, as shown In FIG. 2. Therefore, it is not possible to change the absolute value of the output current when the polarity of the differential input voltage changes. As a result, it is not possible to meet the requirement to change the setting time.